Exactly how much power can a human produce? Scientists want to know.

Back in 1937, researchers at Harvard University’s Fatigue Laboratory published a study they modestly titled “New Records in Human Power.” They’d studied five of the greatest distance runners in the world, including Glenn Cunningham, the fastest miler in history, and Don Lash, the fastest two-miler. The athletes’ ability to suck oxygen into their lungs, absorb it into their blood, then deliver it for use by their muscles—what we would now call their VO2max—was unprecedented. Lash, in particular, was off the charts: his VO2max, in modern units, was 81.4 ml/kg/min, the highest number ever recorded at that time.

Not by coincidence, “New Records in Human Power” is also the title of a review article published last month in the International Journal of Sports Physiology and Performance, by researchers from the Norwegian Olympic Federation and two other Norwegian universities. Since the Harvard study, researchers have occasionally broken through the usual veil of silence surrounding lab measurements of world-class athletes to say something along the lines of “Holy crap, fellow scientists, check out the physiology on this guy!” In the new paper, Thomas Haugen and his colleagues take stock of the current state of knowledge: what do we know about the absolute upper limits of human fitness, circa 2018?

They look at two main categories: maximal aerobic power, a marker of endurance that can be assessed by measuring VO2max; and maximal anaerobic power, a marker of the explosive force produced in activities like jumping and sprinting. I’m going to focus here on the aerobic side, but for the record: the best-of-the-best male athletes can generate about 85 watts of power per kilogram of body weight while jumping, and 36 W/kg while sprinting (running, cycling, or rowing). The top women can generate about 70 W/kg jumping and 30 W/kg sprinting. For context, even professional cyclists can sustain only about 7 W/kg for a five-minute max effort.

For aerobic power, tales of super-high VO2max values have floated around for decades. For example, the mantle of VO2max champion was long granted to Norwegian cross-country skier Bjørn Dæhlie, a 12-time Olympic medalist who reportedly notched a reading of 96 ml/kg/min in the 1990s. When I was researching my recent book, Endure, I got in touch with Stephen Seiler, an American-born sports scientist who has worked in Norway since 1997. He had inspected the data from that test and suspected a calibration problem, in part because the value was 5 points higher than Dæhlie recorded in any other test. That’s a frequent problem with seemingly amazing results: as the new paper points out, most VO2max machines are designed to measure values in hospital patients with abnormally low values, so without special preparation they may not be equipped to handle the prodigious quantities of oxygen that a world-class endurance athlete can breathe.

More recently, an 18-year-old Norwegian cyclist named Oskar Svendsen reportedly tested at 97.5 ml/kg/min in 2012. As if to fulfill the prophecy, he went on to win the junior time trial at the world cycling championships a few weeks later. His VO2max result, like Dæhlie’s, was revealed only in media reports, not in scientific journals.

Tellingly, Haugen his colleagues don’t include either of these measurements in their new paper. (Seiler, a co-author of the paper, shared an early draft with me prior to publication, which is why some of the details are discussed in my book.) Instead, they peg the upper limit of reliably reported values to be about 90 ml/kg/min in men and 80 ml/kg/min in women. Unfortunately, they’re pretty cagey about which athletes exactly are setting these values, with some of the numbers coming from “personal communication with other laboratories testing elite rowers and cross-country skiers.” The women’s records, based on previously unpublished data from Norwegian labs, suggest that three women have recorded values around 80 ml/kg/min: “a long-distance runner (Olympic finalist), an orienteer (Jr. World Championship medalist), and a cross-country distance skier (World Champion).” I’ll leave those clues for others to unravel.

There are a few interesting details to note. One is the difference between “relative” and “absolute” VO2max. The numbers I’ve been quoting are relative values, which means they’re expressed as the amount of oxygen consumed per minute per kilogram of body weight. The absolute value, in contrast, is simply the amount of oxygen consumed per minute, without dividing by body weight. That means that sports like rowing, which are dominated by very large men and women, tend to produce the highest absolute VO2max values, while sports like cycling, with smaller athletes, produce the highest relative values.

Here’s a comparison of the highest reliably observed men’s values for four difference sports, with absolute values on top and relative values on the bottom:

(International Journal of Sports Physiology and Performance)

It’s clear why gigantic rowers have high absolute values and lower relative values. But there are some other subtleties. Why are cross-country skiers so high? Because they’re making full use of both their arms and their legs, which means they have more total muscle extracting oxygen from the bloodstream. Why aren’t there any runners higher than 85 ml/kg/min? Actually, I’m not sure. The authors of the paper also note that this is “somewhat surprising.” I’ll offer a hypothesis, though: the culture of lab testing is much less established among the best distance runners in the world compared to more technical sports like cycling, cross-country skiing, and rowing. I watched Eliud Kipchoge do some treadmill running at Nike headquarters a few years ago, and it was clear that it was a totally unfamiliar experience for him.

The difference between men and women in relative VO2max is typically about 15 percent in elite athletes at comparable performance levels. According to the researchers, that’s mainly because on average women have a higher percentage of body fat (which contributes weight without contributing any oxygen-using capacity) and lower hemoglobin levels to carry oxygen in the blood. The differences in absolute VO2max are also affected by the fact that men simply have bigger bodies, which require proportionately more oxygen.

The unspoken question, of course, is what all these numbers mean. Is there some number beyond which no human may go? In the 1930s, Don Lash’s 81.4 ml/kg/min was the ceiling. Athletes these days train a lot harder; some may also be blood doping or taking drugs like EPO that could artificially raise VO2max, which makes it hard to know exactly how much of the apparent improvement is real. (For the record, I tested at around 82 ml/kg/min twice, once in my 20s and once in my 30s, at two different labs, despite being, at best, a national-class runner—so I know it’s not all drugs.) Conversely, there are also famous examples of great runners with surprisingly low VO2max scores, like Olympic marathon champion Frank Shorter’s 71.3 ml/kg/min.

And then there’s Oskar Svendsen, the teen who set the unofficial VO2max record. After a few underwhelming years as a pro cyclist, he retired from the sport at age 20. It’s fun to look at these records and speculate about what the human body is capable of. But it’s also important to remember that actual races measure something bigger and harder to quantify.

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